Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-) chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g. EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP-A 625728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer latex, as described in e.g. EP-As 770 494, 770 495, 770 496 and 770 497. These patents disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, (2) and developing the image-wise exposed element by applying fountain and/or ink. Another plate that works by latex coalescence is described in EP-A 800928 which discloses a heat-sensitive imaging element comprising on a hydrophilic support an image-forming layer comprising an infrared absorbing compound and hydrophobic thermoplastic particles dispersed in an alkali soluble or swellable resin which contains phenolic hydroxy groups. A similar plate is described in EP-A 1243413 which discloses a method for making a negative-working heat-sensitive lithographic printing plate precursor comprising the steps of (i) applying on a lithographic base having a hydrophilic surface an aqueous dispersion comprising hydrophobic thermoplastic particles and particles of a polymer B which have a softening point lower than the glass transition temperature of said hydrophobic thermoplastic particles and (ii) heating the image-recording layer at a temperature which is higher than the softening point of polymer B and lower than the glass temperature of the hydrophobic thermoplastic particles.
Such image recording layers have also been combined with additional layers as described in EP-A 881096 wherein a heat-sensitive imaging element for making a lithographic printing plate is disclosed which comprises on a lithographic base provided with a hydrophilic surface
(i) an image-forming layer comprising hydrophobic thermoplastic particles and an infared absorbing agent and
(ii) a barrier layer between said image-forming layer and said lithographic base comprises an alkali soluble binder which comprises phenolic or carboxylic groups or phenolic and carboxylic groups.
A recent example of a negative working thermal plate precursor has been described in WO 03/87942 which also comprises a double-layer coating. Latex coalescence, however, has not been disclosed in this document.
The major problems associated with the prior art plate materials which work according to heat-induced latex coalescence are the low run-length which is due to the ease of mechanical damage of the coating of such materials, the low chemical resistance against press liquids and/or the poor image quality.